But in many solid-state triboelectric nanogenerators (TENGs) which continually slide along one another, too much friction leads to damaging wear. Altering the materials or treating the surface can greatly reduce wear, but friction often remains high, which reduces the electric output of the device.
In a paper published in Nano Energy [DOI: 10.1016/j.nanoen.2022.107372], a group of Chinese tribologists and materials scientists report on a TENG inspired by the cylindrical, lubricated roller bearings so ubiquitous in high load rotating machinery.
They started with ABS rollers, experimenting with in three different diameters (1 mm, 3 mm, and 5 mm). They sandwiched a layer of identical rollers between two layers of polyimide (PI) film, adding copper electrodes as the induction layers. The triboelectric pair now assembled, the researchers applied a load to the top film, holding it in in place. The bottom film was moved horizontally, back-and-forth, and the open circuit voltage (Voc) and short circuit current (Isc) measured. With ten ABS rollers in place (d = 1 mm), their rolling friction TENG (RF-TENG) generated a Voc of 219.8 V, almost five times higher than that generated by an equivalent sliding friction TENG. The team then found the electric output was higher with a larger number of larger diameter ABS rollers. The best performance was achieved by using twenty 3 mm rollers; its Voc was 432.7 V. Isc was found to be independent of both the number and diameter of rollers.
In an effort to optimise their generator, they added a small quantity of hexadecane – a commonly-used lubricating compound – onto the rollers, before replacing the top layer of PI film. This had the effect of significantly reducing the friction between the components; by about 85 %, according to their results. The Voc increased significantly too, reaching 1256.7 V. The Isc of this ‘oil-enhanced rolling friction generator’ (ORF-TENG) was also affected by the addition of a lubricant – it increased from 0.55 μA to 5.14 μA. In addition, the ORF-TENG displayed the highest charge transfer of the generators they tested; up to 476.2 nC. By way of a proof-of-concept, they used the TENG to successfully power an array of 50 LEDs.
The authors say that their oil-enhanced generator had an energy conversion efficiency 263 times higher than the equivalent sliding friction TENG. They attribute its enhanced electrical output to “the synergistic effect of the oil molecules and the rolling action of ABS rollers.” They write that “The triboelectric charges could transfer from the rolled ABS cylinder to the oil molecules to further promote the in-situ generation of triboelectric charges, and thereby resulting in the increase of the surface charge density.”
This story was written for Materials Today. It was published on their site on 15th June 2022: https://www.materialstoday.com/energy/news/friction-bearing-inspires-nanogenerator/
Original research paper: Kaiqiang Wang, Caiyang Wu, Hanli Zhang, Jianfeng Li, Jinjin Li. “Cylindrical bearing inspired oil enhanced rolling friction based nanogenerator,” Nano Energy 99 (2022) 107372. DOI: 10.1016/j.nanoen.2022.107372